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serutan writes "Using lasers to drive spaceships has been a subject of interest for many years, but making a photonic engine powerful enough for practical use has been elusive. Dr. Young Bae, a California physicist, has built a demonstration photonic laser thruster that produces enough thrust to micro-maneuver a satellite. This would be useful in high-precision formation flying, such as using a fleet of satellites to form a space telescope with a large virtual aperture. Scaled up, a similar engine could speed a spacecraft to Mars in less than a week."

Are we talking about "accidentally cut Venus in half" scaled up? Typically the downside of photonic thrust has been the low power to weight ratio, so for a laser powerful enough to propel a ship to Mars (don't forget that it has to both accelerate and decelerate) that fast I have to wonder just how powerful the laser has to be.

Are we talking about "accidentally cut Venus in half" scaled up? Typically the downside of photonic thrust has been the low power to weight ratio, so for a laser powerful enough to propel a ship to Mars (don't forget that it has to both accelerate and decelerate) that fast I have to wonder just how powerful the laser has to be.

If you RTFA, you'll note that the quote about reaching Mars in a week doesn't mention anything about a manned mission.

The real question: how the hell are they going to power this laser? For micro-thrusts for satellites, solar panels are fine, but for an interplanetary trip you'd need something like a nuclear reactor (unless that "interplanetary vessel" consisted of a mass of solar panels and a payload about the mass of a postage stamp).

I'd classify this one as just more hype about a technology with an, at present, very limited usefulness. Maybe at some point in the future the human race may use something like this on a large scale. But for now, don't hold your breath.

As I recall, the computations for reaching Mars in a week were predicated on One-G acceleration. i.e. Earth normal gravity for a ship in transit. To slow down, you simply spin the ship at the halfway point and accelerate in the opposite direction.

If (and I stress *if*) this invention is not so much hyperbole, it could change the face of space travel forever. We could build interplanetary starships (in this context, ships that never land on a planet) that would be limited only by their power-generation capabilities and not by their reactive fuel. Which means that we could build a ship with a large nuclear powerplant on board, and it could cruise the solar system for as long as its Uranium/Plutonium fuel held out.

Of course, we still need to solve the problem of high cost of launch, but that little issue would be easier to solve if we actually had somewhere to go once we got in orbit. Scaling up the number of launches would almost certainly bring the price per launch down. In fact, the reason why the Space Shuttle never reached its promised price-per-kilo is because it was predicated on regular launches that never materialized. Starships could change all that. Especially if the cost of moving personnel and equipment was marginalized by carrying more of them per trip.

For example, I always figured that a special module could be fitted to the Shuttle's cargo bay to carry as many as 60 people to the ISS. Given that the Shuttle has to be man-rated for flight, carrying people makes a lot more sense than hauling around equipment that's better served by a Delta or Atlas rocket.

I did quite a bit of reading on spacecraft propulsion recently (specifically Nuclear pulse propulsion [wikipedia.org] and basically what I got out of it is that if you have a massive energy source (say, antimatter) you're better off just blowing it up and riding the blast wave. You can get extremely high thrust AND specific impulse that way, which is not possible with almost any other engine technology (either high thrust and low specific impulse like chemical rockets, or low thrust and high specific impulse like ion engines). NPP (and its derivatives) is basically the best way we know of right now to get high enough performance for interplanetary, or even interstellar, missions.

NPP originally started with using nuclear explosions, but more recent research has focused on inertial confinement fusion and even antimatter-catalyzed fusion. The obvious extreme is using antimatter-matter detonations and riding the blast wave, which I'm fairly certain would be more efficient and yield better performance than taking that energy and pumping it into a laser.

Bingo! 160 km/s somewhere between Earth and Mars absolutely qualifies as solar system escape velocity! I'm a little rusty, but isn't it 400 km/s from the surface of the sun, and around 15 km/s out past Pluto? Voyager II was doing 16 km/s when it left the building...

Since you're using photon pressure, the reaction mass is zero. With sufficient energy, you could travel anywhere in the universe. But unfortunately, Thrust = Power / speed of light.

Even a 1 Newton thruster requires 300 MW at 100% efficiency.

You've gotta scale up the power plant to get more thrust, and it's already going to be pretty massive (I believe that puts it on the order of a medium sized commercial nuke plant.) so I just don't see you reaching Mars in a week. Proxima Centauri in a lifetime, perhaps, but no way on the mars thing.

Of course, since he's talking about a laser, it's possible he means to have the equipment on the ground (or moon, or earth orbit) and propel a much smaller craft. With sufficiently focused optics, you could propel a small probe the whole way to mars (in a week? My envelope just ran out of space...), though it would require some pretty heat-resistant mirrors. Fortunately, the energy requirements for that Newton drop by half when you factor reflection into the equation.

Thrust is the derivative of momentum with respect to time, and momentum is conserved, so in an open-loop drive F=dp/dt=d/dt(E/c)=(1/c)dE/dt, so power (dE/dt) is force times C.

But here's where the novel part comes in. Every photon is bounced back and forth thousands of times between the spacecraft and a mirror. The mirror experiences the same force as the spacecraft but in the opposite direction. The spacecraft's momentum comes from "pushing against" the mirror, rather than "pushing against" the exhaust photons.

For every photon with momentum E/c, the spacecraft gets a momentum kick of E/c when it emits the photon, 2E/c when the photon bounces off it again after a round trip to the mirror, 2E/c again on the next round trip, and so on until the limits of the optics lose the photon out into space. If the drive could really deliver the thousands of photon reuses Dr. Bae talks about, then the power requirements drop to more like 1E10 watts.

You could make a laser out of water ice in orbit to any size using fusion purification and rotation of the billet, doping with chromium or rare earths as you go. Thermal mass should keep it solid enough to pipe light through, and if it's long enough you could add energy slowly enough to pump it to some pretty fantastic numbers of photons before the coherent beam left the less-reflective mirror. Fifty metre aperture? Kilometer in length? Mine the ice from the rings of one of the gas giants and use shaped solar reflectors. You could use silicon too, I imagine, but I like ice because it's cool. Plentiful, too, once we evolve past the point of STS and SFS (Space Food Sticks).

The problem with all of this is scale, right? The energy required to send larger and larger objects would be impractical.

So, what's the smallest thing we can send, then? How small can we make a satellite that can send some information back?

It may not be useful for transporting people to the other end of the universe in a practical amount of time, but I'm sure sending a probe that can check up on Mars every week or so would be of some sort of slight interest to researchers...

His institute seems to have a lot of promising ideas, but no real substance. It has three major projects, one of which relies on the photon thruster and some kevlar straps to toss around satellites, and some sort of undeveloped nano-microscrope.

Well, you can do a back-of-the-envelope calc easily. The mass of the ship is, let's say, about 10 tons or 1E4 kg. You want a 1g acceleration, or about 10 m/s^2 all the way. Assuming a laser with 500nm wavelength a photon leaving will give you an impulse of h/lambda, that is, 6.6E-34 / 5E-7 ~ 1E-27 kg*m/s. Your craft needs to get 1E5 kg*m/s impulse per second to maintain its acceleration, which is then roughly 1E32 photons per second. An 500nm photon has the energy of h * c / lambda, 6.6E-34 * 3E8 / 5E-7 that is ~ 4E-19 J. Thus, all together you need about 4E13 Watts of power, if you have a 100% efficient laser. Now that's about 40TW. Considering that the US produces 4TWh electricity in a year and that a year is about 8760 hours long, you need a power source that is approximately 90 thousand times as powerful as all the power stations of the US put together and it has to fit snugly in your 10 ton rocket, including the fuel. The latter is not as bad as it sounds: if you generate the power by 100% efficient matter-antimatter annihilation the required 40TW power output only needs about a quarter of a gram of each per second, so for a 1-week trip, which is roughly 600,000 seconds, you can get away with about 150kg of each.

So, unless I did a gross miscalculation (entirely plausible) the 1-week Mars flight seems to be a bit out of the realm of reality yet.

But what about the heat? It's quite difficult to cool off lump of metal in a vacuum without discarding hot material to do so. Even if you could feasibly power a craft to Mars with this, how would you stop yourself from arriving as Astronaut McNuggets?

I have my doubts as well. There is a picture there of Dr.Bae standing next to an experimental setup which consists of precision scales, a mirror sitting on these scales, another mirror above it and some sort of laser medium in between.

From this I figure that his thruster uses Fabry-Perot cavity to amplify amount of light circulating between mirrors - not exactly a new trick. However the press release says something about importance of putting laser medium inside the cavity so, hopefully, he is doing something more involved (though not described).

Also, this thrust could only be used to push two mirrors apart - so it is hard to see how one can use this for docking - but undocking would work fine..

Actually, we came pretty close to having a working nuclear rocket engine more than 30 years ago (google for "NERVA" and ignore the Roman emperor links).

The reason we *don't* have such technology today is a result of combining short-sighted congresscritters with techno-illiterate anti-nuclear groups.

To be fair, there *were* some reasonable concerns about the radioactivity of the exhaust (at least while it was near Earth, and there was something in the vicinity to pollute). Using a nuclear reactor to power these new thrusters would alleviate this.

But regardless of this, I suspect that the exact same forces would kill any new attempt at creating a nuclear powered spacecraft. The anti-nuke groups would still be going "Oh Noes - it's nucular, so it's bad". And the congresscritters would still insist on pork from the project for their districts, or else.

So combining these thrusters with a nuclear power source might be *technically* possible, I expect it to remain a political impossibility for the foreseeable future.

If.5G could get you to your turnaround point in 3.5 days, that would mean you'd be going about 1500 km/s when you get there. That's equivalent to 1e6 MJ/kg, or 3.6 MW/kg. Sayth the Wiki [wikipedia.org] that a nuclear fission plant can provide that kind of energy density, and to spare. Not sure about the power density, though, nor about the shielding requirements for human habitation. But from my inexpert viewpoint, the energy requirements look like they'd scale.

I think the problem is that in order to create the propulsion, the laser has to *hit* the craft, not be directed away from it. If I read this correctly, the heat questioned in the grandparent post comes not from powering the laser but from the laser beam smacking against the drive plate.

And given the lack of atmosphere, a heat sink wouldn't help much. The only way to dissipate the heat would be through radiation, and that's slow compared to convection.

The question is, of course, is this really an issue? How much heat is generated from the laser blasting against the drive plate? How quickly will the heat be dissipated?

Of course, since he's talking about a laser, it's possible he means to have the equipment on the ground (or moon, or earth orbit) and propel a much smaller craft. With sufficiently focused optics, you could propel a small probe the whole way to mars (in a week? My envelope just ran out of space...), though it would require some pretty heat-resistant mirrors. Fortunately, the energy requirements for that Newton drop by half when you factor reflection into the equation.

I highly recommend the book Accelerando [wikipedia.org] by Charles Stross, which has an extended story arc which deals with exactly this idea. They're trying to get a coke-can sized space shuttle with a solar sail to a brown start about three light years away (which has an intergalatic router nearby), and they power the shuttle with a laser beam powered by a cable dragged through the jupiter atmosphere/magnetic field. I highly recommend the book. Amazing concepts throughout.

How to scale up.
The original demo was from 10 watt lasers and 3,000 reflections (it is good to actually research original papers to know what is being discussed.)
It is theoretically possible to achieve 100,000 reflections (you may have to go outside the atmosphere to ensure less losses of energy (ie like from a lunar launch system
We will soon be making 100 Kilowatt solid state lasers. (US military made 67 kw earlier this year and will have 100 kw system done later this year or early next year.
We can use arrays of lasers (ie more than one).
Power is provided in electrical form to the lasers. Say from nuclear power (3.2 GW twin reactors, and can have more reactors) or hydro power (Three gorges dam generates 18 GW).
So wattage can go up say 100 million times to 1GW. (reduced the nuclear plant power by inefficiencies for the lasers.
the reflections can increase by 33 times.
Therefore, 3.3 billion times more power.